Photocatalysts are materials having a photocatalytic property of decomposing a substance by irradiation of ultraviolet light, and a photo-induced super-hydrophilic property of making the photocatalyst surface easily soluble in water by exposure to light. Conventionally, photocatalytic self-cleaning products based on these two properties are widely used especially as exterior building materials such as external wall materials, glasses and others.
Specifically, the photocatalytic self-cleaning products can decompose fouling by ultraviolet light present in the spectrum of sunlight and also wash off fouling on surfaces of the products by the super-hydrophilic property when rain falls. Therefore, the products are able to constantly maintain clean appearance (self-cleaning function).
There are several problems in photocatalyst coated self-cleaning glass. Currently, titanium oxide (TiO2) is exclusively used as a photocatalytic material. In order to make a coating film strongly adhesive to a base and obtain a highly durable film, it is necessary to perform firing at high temperature of several hundreds centigrade.
However, since soda-lime glass that is widely used for a window contains a considerable amount of sodium ion, the sodium ion is diffused into the coating layer at the surface of the glass during firing. As a result, a compound of titanium oxide and sodium, like sodium titanate, is formed and the photocatalytic property of the glass is lost.
To avoid such problem of photocatalytic performance loss due to alkali diffusion, two methods are taken in manufacturing of photocatalytic self-cleaning glass.
One of the methods is called a room temperature curing method. In the method, fine particles of titanium oxide photocatalysts are mixed into a coating liquid that is solidified at comparatively low temperature. The resulted mixture is applied to glass and solidified at around 150° C. (see Patent Document 1, for example).
The other of the methods is called a two-layer coating method. In this method, an undercoating film containing a component like silica (SiO2) is formed to inhibit alkali diffusion from a base due to firing. Thereafter, photocatalytic coating and firing are performed (see Patent Document 2, for example).
Patent Document 1: Unexamined Japanese Patent Publication No. 2001-150586
Patent Document 2: Unexamined Japanese Patent Publication No. 10-53439
In the room temperature curing method, since adhesion between glass and a coating film is not high, abrasion resistance of the film is low. This method can only be adopted for very limited use, such as in a part where there is little physical contact with others (like a window of a tall building, for example). In the two-layer coating method, although sufficient coating durability can be achieved, coating has to be performed twice. In addition, according to circumstances, the undercoating film also requires firing. Then, firing steps have to be conducted twice. Cumbersome steps and high costs become necessary.
A similar problem exists in glazed ceramic products and enameled metal products. Glaze and enamel are liquid glasses which are melted or dissolved in a medium. By applying glaze or enamel to a surface of ceramic or metal to become solidified or fired, a glassy film consisting primarily of silicate can be formed. Generally, since it is necessary to add a massive amount of sodium components in order to lower a melting point of glaze and enamel, a massive amount of sodium ion exists in the formed glassy film as in soda-lime glass. Accordingly, in order to form a photocatalytic film on a surface of these products, the room temperature curing method or the two-layer coating method has to be used.